The invention relates to an optical device with reduced birefringence. More particularly it relates to an optical planar device in which the influence of the substrate on the stress-induced birefringence of the optical waveguide device is reduced by modification of the substrate from Underneath the waveguide element.
Integratable planar optical waveguide devices usually consist of a multilayer stack of glass-based materials fabricated onto a suitable substrate. In cases where glass-materials comprise the multi-layer structure, a silicon substrate can be used as the fabrication base, since it is cheap and its processing is well known. Upon application of thermal processing of the multilayer in the fabrication process (growth, diffusion, annealing), the different thermal expansion coefficients of silicon and the glasses lead to unacceptably high values of induced anisotropic stress within the optical guiding structure, notably near the waveguide core. The induced stress changes the propagation characteristics of the TE and TM optical polarizations. With other words, the stress-anisotropy causes birefringence, i.e. polarization-dependent refractive indices. As a practical example, the high refractive index contrast in silica-on-silicon waveguide technology with SiON core layers can be seen. Optical birefringence is a limiting factor in the use and scalability of SiON waveguide technology.
In the article xe2x80x9cCharacterization of Silicon-Oxynitride Films deposited by Plasma Enhanced CVDxe2x80x9d by Claassen, v.d. Pol, Goemans and Kuiper in J. Electrochem. Soc.: Solid state science and technology, July 1986, pp 1458-1464 the composition and mechanical properties of silicon-oxynitride layers made by plasma-enhanced deposition using different gas mixtures are investigated. It is stated that the mechanical stress strongly depends on the amount of oxygen and hydrogen incorporated in the layer. Heat treatment at temperatures higher than the deposition temperature leads to a densification of the film due to hydrogen desorption and cross-linking.
In xe2x80x9cTemperature dependence of stresses in chemical vapor deposited vitreous filmsxe2x80x9d by Shintani, Sugaki and Nakashima in J. Appl. Phys. 51(8), August 1980, pp 4197-4205 its is shown that in vitreous silicate glass depending on deposition background pressure different components of tensile and compressive stress occur. Also a hysteresis of the stress is observed.
In xe2x80x9cStress in chemical-vapor-deposited SiO2 and plasma-SiNx films on GaAs and Sixe2x80x9d by Blaauw in J. Appl. Phys. 54(9), September 1983, pp 5064-5068 stress in films of CVD-SiO2 and plasma-SiNx on GaAs is measured as a function of temperature. Different properties of the stress are observed depending on e.g. film thickness, doping and annealing parameters. xe2x80x9cStress in silicon dioxide films deposited using chemical vapor deposition techniques and the effect of annealing on these stressesxe2x80x9d by Bhushan, Muraka and Gerlach in J. Vac Sci. Technol. B 8(5), September/October 1990, pp 1068-1074 deals with in situ measured stress as a function of annealing temperature. Different deposition techniques are investigated and in PECVD silica films on silicon substrates a change of the stress sign from tensile to compressive is observed with rising annealing temperature.
In U.S. Pat. No. 5,502,781, integrated optical devices which utilize a magnetostrictively, electrostrictively or photostrictively induced stress to alter the optical properties of one or more waveguides in the device are disclosed. The integrated optical devices consist of at least one pair of optical waveguides preferably fabricated in a cladding material formed on a substrate. A stress-applying material, which may be a magnetostrictive, electrostrictive or photostrictive material, is affixed to the upper surface of the cladding material near at least one of the optical waveguides. When the appropriate magnetic, electric or photonic field is applied to the stress applying material, a dimensional change tends to be induced in the stress applying material. The constrained state of the stress applying material, however, caused by its adhesion to the cladding material, causes regions of tensile and compressive stress, as well as any associated strains, to be created in the integrated optical device. By positioning one or more optical waveguides in a region of the device which will be subjected to a tensile or compressive stress, the optical properties of the stressed waveguide may be varied to achieve switching and modulation. Latchable integrated optical devices are achieved by utilizing a controlled induced stress to xe2x80x9ctunexe2x80x9d one or more waveguides in an integrated optical device to a desired refractive index or birefringence, which will be retained after the field is removed.
U.S. Pat. No. 4,358,181 discloses a method of making a preform for a high numerical aperture gradient index optical waveguide. Therein the concentration of two dopant constituents is changed during fabrication. Concentration of the first dopant, GeO2, is changed radially as the preform is built up in order to produce the desired radial refractive index gradient. The concentration of the second dopant, B2O3, is changed radially to compensate for the radial change in thermal expansion coefficient caused by the varying GeO2 concentration. B2O3 is added to the cladding layer to make the thermal expansion coefficient of the cladding equal to or greater than the composite thermal expansion coefficient of the core. The magnitude of residual tension at the inner surface caused by thermal expansion gradients is reduced and premature cracking of the preform is eliminated.
Disclosed in U.S. Pat. No. 4,724,316 is an improved fiber-optic sensor of the type in which a fiber-optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fiber-optic waveguide is altered in response to forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fiber-optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fiber-optic waveguide, the coating material having an expansion coefficient and thickness such that distortion of the fiber-optic waveguide caused by thermally induced stresses between the coating material and the glass fiber is substantially eliminated. Also disclosed is a support member for supporting the curved fiber-optic waveguide, the support member and fiber-optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member.
A reported method to reduce the induced stress within the optical guiding channel is described in U.S. Pat. No. 4,781,424, using the application of grooves adjacent to the channel in order to relieve the stress-component within the glass layers. U.S. Pat. No. 4,781,424 is related to a single mode optical waveguide having a substrate, a cladding layer formed on the substrate, a core portion embedded in the cladding layer, and an elongated member for applying a stress to the core portion or a stress relief groove for relieving a stress from the core portion in the cladding layer along the core portion. The position, shape and material of the elongated member or the groove are determined in such a way that stress-induced birefringence produced in the core portion in accordance with a difference in thermal expansion coefficient between the substrate and the single mode optical waveguide is a desired value. In all methods disclosed therein, the device is subjected to treatment from the upper side, i.e. the side where the waveguide structure is located. The disclosed method further employs a mask to define the grooves and a removal technique to produce the grooves. Both items lead lo significant additional processing work.
In EP 0 678 764 the fabrication of a polarization independent optical device is described. By building the waveguide structure on a silicon substrate, adding a reinforcing layer of glass and removing regions of the silicon substrate underlying the waveguide structure the device is fabricated. The removal of the silicon underlying the waveguide structure eliminates polarization dependent spectral effects by eliminating the source of compressive strain, and the resulting glass reinforced structures are deemed to be sufficiently robust for practical applications.
U.S. Pat. No. 5,483,613 relates to polarization-independent optical devices by reducing or eliminating strain-induced birefrigence associated with prior device structures. An optical device is produced comprising a doped silica substrate having a coefficient of thermal expansion between 8xc3x9710xe2x88x927xc2x0 C.xe2x88x921 and 15xc3x9710xe2x88x927xc2x0 C.xe2x88x921. On the doped silica substrate is formed a doped silica waveguiding structure having a coefficient of thermal expansion between 8xc3x9710xe2x88x927xc2x0 C.xe2x88x921 and 15xc3x9710xe2x88x927xc2x0 C.xe2x88x921. Alternatively, the coefficient of thermal expansion of the doped silica substrate is selected to be approximately 90% to 110% of the coefficient of thermal expansion of the doped silica waveguiding structure. In another aspect, U.S. Pat. No. 5,483,613 provides an optical device comprising a doped silica substrate having a doping gradient from a lower surface to an upper surface. The doping level at the upper surface has a coefficient of thermal expansion approximating the coefficient of thermal expansion of a doped silica waveguiding structure formed thereon.
It is an object of the invention according to claim 1 to provide an optical device with reduced stress on the waveguiding structure or with well-defined stress-values in the region of interest.
It is a further object to provide an alternative solution with regard to the solutions provided in the state of the art for reducing the birefringence in an optical waveguide device.
The optical device with the features according to claim 1 has the advantage that due to the fact that the influence of the substrate is reduced from an area underneath the waveguide, the respective steps for that are less likely to influence the waveguide structure on the substrate. Since the waveguide structure is extremely prone to any post-processing influences like thermal or mechanical influence, it is of utmost advantage that processing steps after completion of the waveguide structure do not or only to a minimum have an effect to it. By treating the substrate from underneath the waveguide structure, the probability of harming the waveguide structure is significantly reduced. Also, etching means which otherwise would even damage the waveguide structure, can be used here because the treatment is done via the underside of the substrate and no contact between the etching means and the waveguide structure occurs. This widens the choice of materials and means for the substrate treatment and reduces risk of damage to the waveguide structure.
Also, treatment from the substrate-underside makes it possible to use spatially unspecific pretreatment of the substrate, i.e. a treatment which concerns the whole wafer underside and has no area-selective component in form of a mask or the like. For instance a mechanical polishing step can be used first, before applying an etching step for more precise or spatially selective substrate removal.
When a layer element with a viscosity lower than the viscosity of the lower cladding layer is arranged between the substrate and the lower cladding layer, the layer element serves for a stress reduction. A strong advantage is the maintenance of planarity and stability of the whole arrangement. Furthermore, the layer element can be manufactured by a simple deposition or material modification process which even does not need a masking step. The layer element can be manufactured by chemically modifying the substrate surface and/or the lower cladding layer and/or an additional layer which has been deposited between the substrate and the lower cladding layer. This provides for a wide variety of modification possibilities which also can be used for optimizing the properties of the layer element with regard to its stress-reducing function as well as mechanical stability and optical influence on the device performance.
The material, respectively viscosity of the layer element, also referred to as float layer, can also be chosen such that in the case of particles residing on the substrate surface which hence render the surface uneven, the float layer serves to replanarize the substrate surface in that it incorporates these particles or grains because of its inherent softness. The waveguide structure to grow on the float layer can then take advantage of a more even surface.
In the dependent claims advantageous modifications and additions to the device of claim 1 are contained.
A ridge-like protrusion element below the waveguide element is very easily manufacturable and furthermore does not reduce the mechanical device stability, because it does not need any different material and makes use of the well-established substrate manufacturing process and eventually the also well known lithography process.
To choose the ridge-like protrusion element to have a width which is such that no additional propagation losses are induced, preferably at least approximately equal to the width of the waveguide element plus twice the thickness of the upper cladding layer, is a choice of geometry which avoids the use of the complex theory of optical waveguidance and uses instead a simple formula which can be easily realized during the ridge-manufacturing process.
When the cladding layers are deposited with a deposition rate which is higher in the direction perpendicular to the upper surface of the ridge-like protrusion element and lower in the direction perpendicular to the sidewalls of the ridge-like protrusion element, the verticality of the sidewalls remains largely unaffected and hence the horizontal dimensions of the ridge-like protrusion element remain as well. This optimizes the destressing effect of the ridge-like protrusion element.
An alternative approach is to choose the substrate to not extend below the waveguiding element. This brings in the advantage that also no additional material is needed to obtain the desired birefringence-reducing effect. The waveguide structure is thereby even decoupled from any potential birefringence-introducing effect from the substrate. This means, that even a substrate made from a material which has a different thermal expansion coefficient than the cladding layers and the waveguide core, can be used without detrimental effect on birefringence.
An advantageous solution is to remove the substrate at least partially in an area below the waveguide element, because this step can be done after finishing the final waveguide layout and manufacturing process. Herefore, no anticipating view of the waveguide structure is necessary, since the removal can be adapted to the finally present waveguide structure. This leaves greater design freedom. Also, for manufacturing the substrate, no complex mask or master structure is needed at the beginning. The substrate is first manufactured as a whole and only afterwards treated with some removing step. A number of removing tools can be used, chemical as well as mechanical ones.
When the area where the substrate has been removed is filled with a filling element which has a thermal expansion coefficient at least approximately identical to the thermal expansion coefficient of the waveguide element, the stability of the arrangement is improved, maintaining the reduced birefringence. The choice of material is wider than with pure substrate modification. The filling element can also be filled in after completing the manufacturing process of the waveguide, including eventual thermal treatment. In this case, the filling element need not have a specific thermal expansion coefficient, since extreme thermal expansion difference is no longer to be assumed, because the filling element is not subjected to the extreme temperatures of the preceding process. The filling element then serves as pure mechanical stabilisator. More generally, the filling element can be an element which has a predetermined expansion coefficient in order to modify the resulting birefringence to achieve an intended value thereof.
When the optical waveguide device is mechanically stabilized, preferably by bonding it to a stabilizing element and/or depositing a stabilizing layer on it, it is subjectable to rougher environmental conditions without risking damage.
Another advantageous solution is chemical modification of the substrate in an area below the waveguide element to form a modified substrate area which has a thermal expansion coefficient at least approximately identical to the thermal expansion coefficient of the waveguide element. This method takes advantage of the stability of the substrate itself and simply adds a modification step which is not effecting material transport in the way, as filling of a depression or creating a gap would do. The modification can be seen as some kind of softening of the substrate in the desired area such that stress does not build up below the waveguide structure.
The birefringence in an optical waveguide is one of the important factors which determines performance of a waveguide type optical component part, so that it is desirable to control the birefringence value with a high degree of accuracy at a desired value.
A key problem of planar waveguide technology is solved, namely the birefringence due to non-isotropic stresses induced during the thermal processing steps. It enables to produce silicon-compatible optical components with very low polarization dependence.
The problem is solved by modifying the substrate from underneath the waveguide structure which keeps the waveguide structure largely unaffected by the modification steps.
The modification comprises the arrangement of a layer element between the substrate and the waveguide structure. Additionally one can build-up of a ridge-like protrusion element under the waveguide core, remove completely or partially the substrate under the waveguide core, or chemically modify completely or partially the substrate under the waveguide core. The ridge, layer element and chemical modification can be performed even before the waveguide structure is formed such that these birefringence-reducing measures can not affect the device performance. The last steps in manufacturing the optical waveguide device are then only the creation of the upper cladding layer and eventually a thermal treatment, like an annealing step which steps are the only ones which are to be done after manufacturing the waveguide structure.
In practice, a combination of the different stress-reduction techniques is envisaged. In this way the different removal rates and selectivities of the various techniques are exploited.